A geographical area covered by a cellular wireless telecommunications network may be separated into a patchwork of smaller geographical areas or cells, which are each served by a base station. Each base station communicates with end user equipments, which subscribe to the network operator of the network and which are located within the cell served by the base station. A network controller coordinates the transmission of messages, comprising for example voice or data, via the base stations. For optimum coverage over a cell, the base station may transmit and receive messages over one or more Frequency Division Duplexes (FDDs). In a FDD a first channel at a first frequency is used constantly as a downlink, for carrying messages from the base station to user equipments within the cell. A second channel, at a different frequency from the first is used constantly as an uplink for carrying messages from the user equipments within the cell to the base station.
However, there can be a problem that some user equipments within the cell may not be able to communicate with the base station over the FDD because for example, there is an obstacle between the base station and the end user equipment. For example, it may be that user equipments located on an upper floor of a building can communicate with the base station, whereas user equipments located on a tower floor or in the basement of the building cannot communicate with the base station. One way of solving this problem is to install repeater units adjacent to regions of a cell where coverage is poor. The repeater units, receive messages from the base station and re-transmit them to the user equipments and receive messages from the user equipments and retransmit them to the base station. As the repeaters transmit and receive the messages at a location closer to the user equipments than the base station, user equipments in the poor coverage region of the cell have a good chance of establishing communication with the base station via the repeater. However, repeater units are relatively expensive and the deployment of repeaters in regions with high user density, such as urban environments, can be prohibitively expensive because of the shortage of sites for the location of repeater units and the high rental costs for such locations. Also, the deployment of repeater units in regions of low user density, such as rural environments, is not cost effective as the numbers of user equipments in cell regions with poor coverage is generally too low.
Investigations have also been carried out to try to improve capacity within cells. One way to do this is to use one or more base station equipments operating in Time Division Duplex (TDD) mode in addition to the FDD mode. In the TDD mode the same frequency channel is used for communications from the base station to the user equipments and from the user equipments to the base station, with the direction of transmission of messages on the channel varied in a controlled way so that the transmission of messages at any given time is one way.
However, TDD is not ideally suited for wide area coverage. In order to improve coverage within the poor coverage regions of a cell having a TDD infrastructure, relatively simple TDD relays can be deployed. The TDD relays may include relay units specifically deployed by the network operator of user equipments and/or end user equipments equipped with a TDD relay capability. Simple relays and user equipments can be used because TOO is ideally suited to relay because the same channel can be used for receipt and re-transmission of the relayed message, with the receipt and re-transmission separated in time so that the TDD for the relay has changed from a receive mode to a transmit mode.
The Situation is more complicated where the base station has a FDD infrastructure. This is because for an FDD repeater the uplink and downlink channels must be reversed in frequency so as to ensure that a standard end user equipment can be used. This frequency swapping creates the potential for self interference where the repeater transmitter may cause interference to the repeater receiver. Alternatively, the FDD repeater can map the FDD system into another spectrum allocation to avoid self interference, however, this requires the end user equipment to be multi-band and still the repeater may be interfered with or cause interference to other systems. Therefore, it has been suggested in order to extend high bit data rate FDD coverage to the periphery of a cell, which periphery can support only low bit data rate FDD coverage, that a static relay node may be deployed by the network operator at the border between the high bit data rate coverage area and the periphery of the cell. The static relay node would receive a message at the high hit data rate over a base station FDD and relay it, at the high bit data rate, on a TDD towards its destination. Although FDD/TDD repeaters will be less prone to the self interference effects (the TDD onwards transmission will be, at worst, in an adjacent band, rather than in-band as is the case for co-channel FDD) they suffer similar disadvantages to FDD only repeaters. Fixed repeater installations of all types are likely to attract similar site location and rental issues.